Calderas form during massive volcanic eruptions where large portions of the surface collapse into the emptying magma chamber below. This type of eruption generates hundreds of cubic miles of ash, which in turn creates tremendous amounts of volcanic rock.

Pictured right:

  • Upper image shows the initial phase of an idealized caldera ash flow eruption.
  • Lower image shows subsequent collapse of overlying rock into themagma chamber below forminga depression-like featurecalled a”caldera”.

Colorado has experienced at least 20calderaeruptions.The San Juan volcanic field is one of the most notable areas in Colorado where this type of eruption occurred. Hundreds of square miles of Colorado’s southwestare covered by ash flow tuff generated by these eruptions. The world’s largest caldera, La Garita, is also located in the SanJuan volcanic field. This calderaextruded over 1,200 cubic miles of ashflow, which is also the world’s largest ash flow deposit.

Below: Locations of several Calderas (beige) along with other volcanics (red) in Colorado. Note the large cluster of calderas in the southwest whichlie withinthe San Juan Volcanic Field.Within La Garita’s outline, multiple smaller calderas were active post La Garita’s eruption.

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Initial phase of an idealized caldera ash flow eruptionSubsequent collapse of overlying rock into magma chamber in an idealized caldera ash flow eruption

Distribution of calderas in Colorado